Natural gas hydrate is ice‐like mixture of gas (mostly methane) and water that is widely found in sediments along the world's continental margins and within and beneath permafrost and glaciers in a near‐surface depth interval where the pressure is sufficiently high and temperature sufficiently low for gas hydrate to be stable. We categorize the myriad of geological gas hydrate deposits into five characteristic types. We then review the multiple quantitative models that have proposed to describe the genesis of these deposits and describe how each may have formed. We emphasize the importance of coupling multiphase flow (free gas and liquid water) and multicomponent reactive transport with geological history to describe the dynamical processes of gas hydrate formation and evolution in geological systems. A better insight into the kinetics of methane formation from microbial biogenesis and the processes of multiphase flow at the pore scale will advance our knowledge of how these systems form. By understanding the generation and evolution of gas hydrate through time, we will better decipher the role of gas hydrate in the carbon cycle, its potential to contribute to climate change and geohazards, and how to design optimal strategies for gas production from hydrate reservoirs.

[1] We present laboratory experiments designed to quantify the stability and energy budget of buoyancy-driven iceberg capsize. Box-shaped icebergs were constructed out of low-density plastic, hydrostatically placed in an acrylic water tank containing freshwater of uniform density, and allowed (or forced, if necessary) to capsize. The maximum kinetic energy (translational plus rotational) of the icebergs was $15% of the total energy released during capsize, and radiated surface wave energy was $1% of the total energy released. The remaining energy was directly transferred into the water via hydrodynamic coupling, viscous drag, and turbulence. The dependence of iceberg capsize instability on iceberg aspect ratio implied by the tank experiments was found to closely agree with analytical predictions based on a simple, hydrostatic treatment of iceberg capsize. This analytical treatment, along with the high Reynolds numbers for the experiments (and considerably higher values for capsizing icebergs in nature), indicates that turbulence is an important mechanism of energy dissipation during iceberg capsize and can contribute a potentially important source of mixing in the stratified ocean proximal to marine ice margins.

We present a one-dimensional model that couples the thermodynamics of hydrate solidification with multiphase flow to illuminate how gas vents pierce the hydrate stability zone in the world's oceans. During the propagation phase, a free-gas/hydrate reaction front propagates toward the seafloor, elevating salinity and temperature to three-phase (gas, liquid, and hydrate) equilibrium. After the reaction front breaches the seafloor, the temperature gradient in the gas chimney dissipates to background values, and salinity increases to maintain three-phase equilibrium. Ultimately, a steady state is reached in which hydrate formation occurs just below the seabed at a rate necessary to replace salt loss. We show that at the Ursa vent in the Gulf of Mexico, the observed salinity and temperature gradients can be simulated as a steady state system with an upward flow of water equal to 9.5 mm yr À1 and a gas flux no less than 1.3 kg m À2 yr À1 . Many of the world's gas vents may record this steady state behavior, which is characterized by elevated temperatures and high salinities near the seafloor.

Key Points:• Ice melange experiences widespread jamming during calving events • The ice melange is closely packed during periods of terminus quiescence • The kinetic energy of the melange is about 1% of the total released energy Supporting Information:• Text S1• Movie S1• Movie S2 Abstract We investigate the dynamics of ice mélange by analyzing rapid motion recorded by a time-lapse camera and terrestrial radar during several calving events that occurred at Jakobshavn Isbrae, Greenland. During calving events (1) the kinetic energy of the ice mélange is 2 orders of magnitude smaller than the total energy released during the events, (2) a jamming front propagates through the ice mélange at a rate that is an order of magnitude faster than the motion of individual icebergs, (3) the ice mélange undergoes initial compaction followed by slow relaxation and extension, and (4) motion of the ice mélange gradually decays before coming to an abrupt halt. These observations indicate that the ice mélange experiences widespread jamming during calving events and is always close to being in a jammed state during periods of terminus quiescence. We therefore suspect that local jamming influences longer timescale ice mélange dynamics and stress transmission.